Take the following into consideration when choosing between copper and plastic:
Price: 15mm copper £2.05/mtr, 15mm plastic £0.90/mtr (prices correct at time of writing - please check the Plumbing & Sundries section for up to date pricing)
Saving on materials: No gas, solder, soldering mat, flux, etc.
Corrosion in copper - Hep2o has a 50 year guarantee!
Less fittings required: The pipe can be used similarly to cable as in it can be pulled through joists for example, saving on joints every few metres or so.
Time: Push the fitting on with the insert in the pipe and thats it!!
Why use plastic plumbing you ask??!!